well, many programs are willing to at least try to open anything you throw at them e.g. notepad, most of MS office suite; the results may be useless but they at least try.

And what's this nonsense about "protecting sigil" from viruses. the only thing at risk here is the user's PC, nothing is going to zap across cyberspace & infect the program authors! If the right kind of virus is loose on a pC it can infect sigil or any other exe with malicious add-ons anyway. it does not matter what formats sigil is set to open as infection can happen to exe files that are not in active use, just on the local hard drive.